Fork lift trucks and other material handling vehicles place great strain upon their motor systems during hard braking and maximum accelerating and decelerating conditions. It is during these operational conditions that the DC motor of the fork lift vehicle experiences peak current conditions in the armature. It is important to limit the peak currents flowing through the DC armature in order to reduce heating therein and to improve the efficiency of the operation of the fork lift truck.
It is most common to apply plug-braking in DC motors used in material handling vehicles. Such a technique is illustrated in U.S. Pat. No. 3,826,962, issued to Morton et al on Jul. 30, 1974, entitled "Control of Electric Motors for Battery-Operated Vehicles." This system avoids overheating when "regeneration" or plug-braking is utilized. A coil is used to sense plugging current which is then used to energize a contactor. The field current is regulated to keep the generated current constant as motor speed slows, thereby providing constant braking torque. In the DC motor of this patented system, the heating effects are small. This is particularly true when contrasted to DC motors employed in fork lift truck operations.
When a fork lift vehicle plug-brakes, it brings a 12,000 pound vehicle to a stop in an average of three seconds. This dynamic stopping condition equates to 19,750 ft-lbs of energy, or 26,800 joules. This energy must be dissipated almost entirely in the armature circuit. This is a great amount of energy to be dissipated and would ordinarily cause severe heating in the armature of the DC motor, were it not for the present invention.
It should also be observed that the aforementioned braking condition of the vehicle is on the average occurring 100 times every hour of operation. When this enormous amount of heat energy is also accompanied by other losses during normal driving and acceleration, it is no wonder that this serious problem must be eliminated or at least effectively controlled.
The present invention controls peak armature DC motor current during the entire operation of the vehicle, one hundred percent of the time. A current sensor is utilized to provide both magnitude and polarity of armature current. A processor utilizes this information to control both a switching contactor and pulse switching means at an accelerated rate that only computer processing can handle. The inventive control system eliminates or removes as much heat from the armature during plug-braking as possible and yet provides an adequate braking effort to the vehicle.
In prior art systems, velocity control or constant braking force limitations have been placed upon the operational system to improve efficiency of the operation. However, none of these systems has regulated current through the armature of the DC motor for the purposes of this invention, viz., protecting the armature from overheating. A prior art system that used velocity feedback to regulate applied power for acceleration and deceleration is shown in U.S. Pat. No. 3,466,524, issued to Cooper on Sep. 9, 1969, for a "Speed Taper Brake Modulation System." In this system, the braking effort is controlled as a function of speed. A special motor with an extra winding is utilized for controlling motor impedance and thus current. This is equivalent to a variable speed DC motor system.
In the aforementioned patent to Morton et al, the contactor and resistor control is analogous to an on/off light switch. By contrast, the present invention utilizes an SCR dimmer control. Whereas the Morton et al system merely switches on and off depending upon the presence of plugging current, the present invention senses the motor current with its polarity changes almost instantly. The invention uses this information to switch the contactor "cold", with little or no current passing through the tips. The plugging current is directed through a resistor. The pulse width and frequency of the current is precisely varied at the SCR switch to regulate the current through the armature.
The advantages of the present invention are that 75 to 80% of the energy once dropped in the armature is now dissipated by the system's external resistor during plug-braking. This greatly reduces the operating temperature of the motor, which experiences this braking condition on the average of 100 times per hour.
Contactor tip wear is also greatly reduced due to precise switching of the contactor as a function of armature current, wherein little or no current passes through the tips during the braking.
The system removes the resistor from the circuit during conditions that do not require plug-braking. This increases the recirculating current during normal acceleration and driving. The result is that the delivered net torque of the motor is improved. Thus, the entire system is more efficient over its entire operation.